This invention relates to a developing method or a developing bristle height measuring method and apparatus practicing the methods as well as color image printers.
Various types of color toner image printing method and color toner image printer using electrophotography have been proposed; and from the standpoint of speed-up and continuous paper recording specification, method and apparatus of the type wherein a color toner image is formed on the surface of a rotating sensitive member (sensitive drum or sensitive belt) and the color image is transferred to a recording paper are highly evaluated.
In this type of printer using, for example, a sensitive drum, a plurality of pairs of electrostatic latent image forming means and developing means are arranged in the direction of rotation of the sensitive drum to oppose the same and development is repeated plural times in order to form toner images of a plurality of kinds of color at one image forming area. In this printer, however, development for the second color is done at the surface of the sensitive drum where a first color toner image is formed; and, if a developing means of contact type is used for the second color, the first color toner image is scraped off by the developing means for the second color and decreased in image density, and besides the scraped-off first color toner intrudes into the second color developing means, causing color mixing. This problem also takes place in the third and the following developing means.
Approaches to this problem have been proposed wherein a contactless magnetic brush or a magnetic brush of weak slide-contact force is used for the succeeding developing means.
This type of printer is disclosed in, for example, Japanese Patent Publication No. 63-43748, JP-A-52-106743 and JP-A-56-144452.
The prior art printer, however, does not consider how the developing gap should be related to the bristle height of a magnetic brush for the purpose of obtaining excellent color images, and it faces difficulties in insuring sufficient image density of both the first and second color toner images. More specifically, the maintenance of image density of the first color toner image tends to cause a decrease in image density or irregularity in density of the second color toner image and conversely, the maintenance of image; density of the second color toner image tends to cause occurrence of fog or a decrease in image density of the first color toner image. Disadvantageously, the prior art apparatus does not consider a change in image density due to a slight change with time in developing conditions, either and is difficult to maintain quality of printed images stably.